Paper is sized to resist the penetration of liquids. This invention is concerned with the sizing of paper to resist penetration by water and aqueous solutions.
Paper can either be surface sized or it can be sized internally. Internal sizing is accomplished by the application of a sizing agent to the pulp slurry prior to formation of the paper sheet. Surface sizing, on the other hand, entails application of the sizing agent to the surface of a formed paper sheet. Internal and surface sizing are discussed in J. P. Casey, Pulp & Paper, Chemistry and Chemical Technology, Second Edition, Volume II, Interscience Publishers, Inc., New York, 1960.
For well over a century, internal sizing operations have employed rosin in combination with alum. Various improvements in the rosin-alum treatment have been made.
More recent internal sizing agents include wax emulsions, stearates, alkylketene dimers and alkenyl succinic anhydrides. Most of these new sizes are described in TAPPI Monograph Series 33, "Internal Sizing of Paper and Paperboard," Mack Printing Company/Easton, PA, 1971.
Current commercial internal sizing agents have important individual drawbacks. For example, rosin requires alum and an acid pH in papermaking. Alkylketene dimers give best performance in a non-acid system. Alkenyl succinic anhydrides cannot be produced as stable emulsions and require special emulsification at the site of use. Some other sizes are less efficient or have other particular drawbacks.